The present invention relates generally to PTFE dispersion polymerization techniques, and particularly to a new class of surfactants for use in PTFE dispersion polymerization.
Polytetrafluoroethylene homopolymer (PTFE) and copolymers of tetrafluoroethylene (TFE) and ethylenically unsaturated comonomers can be produced in aqueous media by two distinctly different procedures. In one procedure, little or no dispersing additives, or surfactants, are added, and vigorous agitation is provided, yielding a precipitated resin, usually referred to as "granular resin." In the second procedure, a dispersing additive or surfactant is added, and mild agitation provided, to yield a uniform suspension of small polymer particulates. The products of the two processes are employed in different applications; granular resin is fabricated into products by molding, while the resin produced by dispersion polymerization is employed in dispersion coating and paste extrusion fabrication processes.
Numerous classes of suitable surfactants for PTFE dispersion polymerization have been proposed and evaluated. Included within these classes are polyfluoroalkanoic acids and salts thereof; polyfluorosulfonic acids and salts thereof; polyfluorophosphonic acids and salts thereof; sulfuric and phosphoric acid esters of polyfluoroalkanols; and polyfluoroalkylamine salts. These classes and other suitable dispersing agents are disclosed by Berry, U.S. Pat. No. 2,559,752, among others. Other useful general references relating to PTFE dispersion polymerization processes include Punderson, U.S. Pat. No. 3,391,099; Porter, U.S. Pat. No. 3,704,285; Gangal, U.S. Pat. No. 4,186,121; and Khan, U.S. Pat. No. 4,380,681.
Of the foregoing classes of surfactants, the most widely employed are sodium and ammonium salts of a perfluoromonocarboxylic acid having 6 to 10 carbons, especially ammonium perfluorooctanoate.
The foregoing classes of surfactants can be structurally characterized as bifunctional compounds having regions of different polarity; specifically, each surfactant molecule comprises a hydrophobic group and a hydrophilic group. In dispersion polymerization processes, the hydrophobic group is adsorbed onto the surface of a growing particle, while the hydrophilic group remains in the aqueous phase. The hydrophilic group is hydrated in the aqueous medium and in the case of ionic surfactants, it is also ionized, imparting a charge to the particle surface. As a result of their like-charged surfaces, individual particles repel each other, preventing coagulation. This combination of properties in a surfactant improves dispersion stability and favors efficient polymerization.
Alkoxides have not been previously proposed as potential surfactants, since these compounds are readily hydrolyzed by water in an equilibrium reaction, for example: EQU RO.sup.- +H.sub.2 O.revreaction.ROH+OH.sup.-
However, in the case of tertiary perfluoroalkoxides, the equilibrium favors the alkoxide, enabling use of such compounds as surfactants. Moreover, it has now been found that use of tertiary perfluoroalkoxides as surfactants provides unexpected increases in polymerization rates. Heretofore, increases in polymerization rates, and hence process productivity, could be obtained only by increasing concentrations of polymerization initiator. However, this expedient provides polymer having reduced average molecular weight. Use of tertiary perfluoroalkoxides as surfactants provides significantly increased polymerization rates without adversely affecting the size of polymer chains or the morphology of the resulting dispersed polymer particles.